Biomechanical implications of mass loading in a swine model of acute hypoxemic respiratory failure.

In obesity, excess weight of the chest and abdomen (mass loading) decreases lung volume and can worsen acute hypoxemic respiratory failure (AHRF). We investigated whether positive end-expiratory pressure (PEEP) fully reverses the effects of mass loading on lung volume and respiratory mechanics in an AHRF swine model. Eighteen Yorkshire pigs were studied: six healthy, eight pre- and postinjury, and four postinjury only. We randomly tested three mass loading conditions: without mass loading, with abdominal loading (6 kg weight), and with combined abdominal and chest mass loading (12 kg total weight). We performed a recruitment maneuver in each condition, followed by a decremental PEEP trial, and identified the best-PEEP as that with the greatest respiratory system compliance (CRS). Airway pressure, esophageal pressure, and thoracic impedance by electrical impedance tomography) were continuously monitored. After lung injury, best-PEEP increased with loading. CRS at best-PEEP decreased from 20.6 ± 3.4 mL/cmH2O without loading to 17.7 ± 3.0 mL/cmH2O with abdominal loading [mean difference 2.9, 95% confidence interval (CI): 1.6-4.2] and to 14.2 ± 2.8 mL/cmH2O with abdominal and chest loading (mean difference 6.3, 95% CI: 5.0-7.7). Any amount of loading decreased end-expiratory lung volume assessed by computed tomography (CT) at best-PEEP and PEEP 3 cmH2O. Combined abdominal-chest loading decreased the vertical lung dimension on CT compared with unloaded and abdominal loading at both levels of PEEP. With mass loading, PEEP did not restore values of CRS and lung aeration to their unloaded values. In AHRF with mass loading, geometrical constraints may limit PEEP efficacy even when optimally titrated.NEW & NOTEWORTHY In a ventilated swine model, we isolated the mechanical effects of mass loading from those of lung injury. Despite optimization, positive end-expiratory pressure (PEEP) could not restore baseline pulmonary compliance or lung volumes in either healthy or injured lungs. Partitioned respiratory mechanics and imaging reveal that geometric constraints and load-induced airway closure, demonstrated here for the first time in injured swine, may limit the effectiveness of recruitment maneuvers and optimized PEEP in mass-loaded lungs.